The present invention relates to an improved process for preparing a substituted or unsubstituted phenylcyclohexane by catalytic hydrogenation of a substituted or unsubstituted biphenyl.
The preparation of phenylcyclohexane by full hydrogenation of one phenyl ring of the biphenyl is known in principle.

By-products formed usually always include, inter alia, bicyclohexyl which forms an azeotrope with phenylcyclohexane.
In Journal of the Chemical Society 1951, pages 1371-1372, I. Goodman describes the preparation of phenylcyclohexane by catalytic hydrogenation of biphenyl to phenylcyclohexane in ethanol in the presence of Raney nickel.
Tetrahedron Letters 2000, No. 41, pages 5865-5868, describes the preparation of phenylcyclohexane by hydrogenation of biphenyl in the presence of Raney nickel-aluminum alloys at a temperature of 90° C.
DE 937 950 discloses a process for the catalytic hydrogenation of biphenyl to phenylcyclohexane using a nickel- and cobalt-free copper-chromium catalyst at a temperature of about 240-260° C.
U.S. Pat. No. 3,387,048 discloses a process for preparing phenylcyclohexane by hydrogenation of biphenyl with addition of the solvent cyclohexane. A suitable catalyst is indicated as being 5% palladium on carbon.
DE 2 125 473 relates to catalysts comprising cobalt oxide or a mixture of cobalt oxides, e.g. Co3O4 and CoO, for the partial hydrogenation of biphenyl derivatives.
WO 93/16972 relates to catalysts for the hydrogenation of polycyclic and monocyclic aromatics such as alkyl-substituted benzene derivatives and substituted biphenyls, with the catalysts comprising ruthenium.
CN 1800121 discloses a process for preparing phenylcyclohexane by hydrogenation of biphenyl in the presence of nickel-aluminum catalysts.
EP 0 394 842 relates to catalysts for the hydrogenation of aliphatic unsaturated compounds, where the catalysts comprise nickel and copper and are characterized by a content of from 20 to 75% by weight of nickel oxide, from 10 to 75% by weight of zirconium dioxide and from 5 to 50% by weight of copper oxide, in each case based on the oxidic, unreduced catalyst. The catalysts are particularly suitable for the hydrogenation of the industrially important compounds 2-butyne-1,4-diol, 2-butene-1,4-diol and 2-ethylhexen-2-al.
Some of the above-described processes for preparing substituted or unsubstituted phenylcyclohexane use catalysts which are either not easy to handle on an industrial scale or are not readily available. Furthermore, the known processes in some cases display an unsatisfactory selectivity to the target product and/or a space-time yield which is too low.